1. Field of the Invention
The present invention relates to a switching power supply apparatus used as a direct-current power source for an electronic appliance, and more particularly to a switching power supply apparatus provided with an overcurrent protection circuit that protects the switching power supply apparatus from an excessively large output current.
2. Description of the Prior Art
Conventionally, switching power supply apparatuses are provided with an overcurrent protection circuit to prevent their destruction resulting from an excessively large output current, for example, when the output terminals are short-circuited together.
Protection against an overcurrent by means of an overcurrent protection circuit is realized in the following manner. According to one method, called the shut-down method, when a switching power supply apparatus enters a predetermined overcurrent state, switching operation is stopped, and, even when the overcurrent state is cancelled, switching operation is not restarted automatically, but it is only when the power to the switching power supply apparatus is turned off first and then on that the switching power supply apparatus restarts switching operation. According to another method, called the automatic recovery method, canceling the overcurrent protection state causes the switching power supply apparatus to restart switching operation automatically. Which of these methods to adopt is determined at the time of designing a switching power supply apparatus according to the characteristics of the electronic appliance that is fed with power by the switching power supply apparatus and the user's preference.
FIG. 10 is a circuit diagram of a conventional switching power supply apparatus that adopts the automatic recovery method. In FIG. 10, commercially distributed alternating-current power (not shown) is connected to alternating-current power input terminals 1 and 2. Between the alternating-current power input terminals 1 and 2, there is connected, by way of a filter 3, a bridge rectifier circuit 4. The filter 3 is composed of a capacitor 3a, line filter coils 3b, and a capacitor 3c. The bridge rectifier circuit 4 is composed of diodes 4a, 4b, 4c, and 4d. The filter 3 and the bridge rectifier circuit 4 are connected together by lines L1 and L2, and between these lines L1 and L2 is connected a discharge resistor 31.
The output ends of the bridge rectifier circuit 4 are connected to a positive power supply line L3 and a negative power supply line L4, respectively, and between these lines L3 and L4 are connected a capacitor 5, a serial circuit composed of a primary coil 6a of a transformer 6 and an FET (field-effect transistor) 7, and a serial circuit composed of a resistor 15 and a capacitor 17. The FET 7 functions as the main switching device of this switching power supply apparatus. One end of an auxiliary coil 6c of the transformer 6 is connected through a diode 16 to the node between the resistor 15 and the capacitor 17, and the other end of the auxiliary coil 6c is connected to the negative power supply line L4.
A switching control circuit 14 has a positive power input terminal thereof connected to the cathode of the diode 16, has a negative power input terminal thereof connected to the negative power supply line L4, has a feedback input terminal thereof connected to the collector of a phototransistor 13b of a photocoupler 13, and has an output terminal thereof connected to the gate of the FET 7.
One end of a secondary coil 6b of the transformer 6 is connected through a diode 8 to a positive output line L5, and the other end of the secondary coil 6b is connected to a negative output line L6. Between the positive and negative output lines L5 and L6, there are connected a capacitor 9, a serial circuit composed of a photodiode 13a of the photocoupler 13, a resistor 12b, and a shunt regulator 12a, and a serial circuit composed of resistors 12d and 12c. The node between the resistors 12d and 12c is connected to the monitoring terminal of the shunt regulator 12a. The shunt regulator 12a and the resistors 12b, 12c, and 12d together constitute an output voltage detection circuit 12.
The positive output line L5 is connected to a positive output terminal 10, and the negative output line L6 is connected through an output current detection circuit 18 to a negative output terminal 11. The control terminal of the output current detection circuit 18 is connected to the node between the photodiode 13a and the resistor 12b. The output current detection circuit 18 may be provided between the positive output line L5 and the positive output terminal 10.
Next, the operation of this conventional switching power supply apparatus will be described. When commercially distributed alternating-current power (not shown) is fed to the alternating-current power input terminals 1 and 2, it is fed through the filter 3 to the bridge rectifier circuit 4 and is thereby rectified. The rectified voltage is then smoothed by the capacitor 5 and is thereby converted into a direct-current voltage. This direct-current voltage is fed, as operating power for the main circuitry of the switching power supply apparatus, to the positive and negative power supply lines L3 and L4.
The direct-current voltage fed between the positive and negative power supply lines L3 and L4 makes the switching control circuit 14 operate so as to make the FET 7 perform switching operation. As a result, a high-frequency voltage is induced in the secondary coil 6b of the transformer 6, and this high-frequency voltage is rectified and smoothed by the diode 8 and the capacitor 9 and is thereby converted into a direct-current voltage. This direct-current voltage is fed, via the positive and negative output terminals 10 and 11, to an electronic appliance (not shown) that serves as a load.
The voltage between the positive and negative output lines L5 and L6 is divided by a voltage division circuit constituted by the serially connected resistors 12d and 12c, and the divided voltage is fed, as a monitored voltage, to the monitoring terminal of the shunt regulator 12a. The shunt regulator 12a compares the monitored voltage fed to the monitoring terminal thereof with a reference voltage previously set therein, and feeds a current commensurate with the result of the comparison to the photodiode 13a of the photocoupler 13 to make the photodiode 13a emit light.
The light from the photodiode 13a is received by the phototransistor 13b of the photocoupler 13, and the phototransistor 13b feeds, as a feedback signal, a voltage commensurate with the result of the aforementioned comparison to the feedback input terminal of the switching control circuit 14. The switching control circuit 14 controls, according to the feedback signal thus fed thereto, the switching operation of the FET 7 so as to stabilize the output voltage of the switching power supply apparatus.
When the switching power supply apparatus starts to start up, the switching control circuit 14 starts to operate from the current fed from the positive terminal of the capacitor 5 through the start-up resistor 15. By contrast, when the switching power supply apparatus is operating in a steady state, the switching control circuit 14 operates mainly from the direct-current power produced by rectifying and smoothing, with the diode 16 and the capacitor 17, the voltage induced in the auxiliary coil 6c of the transformer 6.
The output current detection circuit 18, which is connected between the negative output line L6 and the negative output terminal 11, compares the current on the negative output line L6 with a reference current previously set therein. When the current on the negative output line L6 is larger than the reference current, the output current detection circuit 18 short-circuits together the cathode of the photodiode 13a of the photocoupler 13 and the negative output line L6. This increases the current through the photodiode 13a. In a case where the output current detection circuit 18 is connected between the positive output line L5 and the positive output terminal 10, the current on the positive output line L5 is compared with the aforementioned reference current.
When the switching control circuit 14 is fed, through the phototransistor 13b, with information on this increase in the current in the form of a feedback signal, the switching control circuit 14 recognizes that the output voltage of the switching power supply apparatus has increased greatly, and thus controls the switching operation of the FET 7 in the direction in which the output power of the switching power supply apparatus decreases.
FIGS. 11A to 11C are voltage waveform diagrams illustrating the operation of the switching power supply apparatus at start-up. Now, with reference to these voltage waveform diagrams, how the switching power supply apparatus operates at start-up will be described.
When commercially distributed alternating-current power is connected between the alternating-current power input terminals 1 and 2 at a time point TO shown in FIGS. 11A to 11C, a start-up current is fed from the positive terminal of the capacitor 5 through the start-up resistor 15 to the capacitor 17, and this causes the charge voltage Vcc of the capacitor 17 to increase gradually as shown in FIG. 11A. When the charge voltage Vcc reaches the operation start voltage of the switching control circuit 14 at a time point T1, the switching control circuit 14 starts to feed a drive signal to the FET 7. As a result, the switching power supply apparatus starts to start up, and thus the output voltage Vo of the switching power supply apparatus (i.e., the voltage between the output terminals 10 and 11) starts to increase as shown in FIG. 11B. The output voltage Vo reaches the target output voltage of the switching power supply apparatus at a time point T3.
After the time point T1, an induced voltage appears in the auxiliary coil 6c of the transformer 6 as shown in FIG. 11B, and the voltage level of this induced voltage in the positive direction increases in proportion to the output voltage of the switching power supply apparatus until, at a time point T2, it becomes equal to the voltage level of the charge voltage Vcc. Now, a current produced by rectifying the included voltage with the diode 16 flows into the capacitor 17, and thus the charge voltage Vcc starts to increase as shown in FIG. 11A. After the time point T3, i.e., once the target output voltage of the switching power supply apparatus is reached, the charge voltage Vcc stabilizes at a fixed voltage proportional to the output voltage.
As shown in FIG. 11A, during the period from the time point T0 to the time point T1, the charge voltage Vcc increases owing to the current fed through the start-up resistor 15 because, during that period, the switching control circuit 14 is not operating and thus consumes only a small current. However, when the switching control circuit 14 starts to operate at the time point T1, the current consumed by the switching control circuit 14 becomes larger than the current fed through the start-up resistor 15. This causes the charge voltage Vcc to start to decrease. Then, at the time point T2, the charge voltage Vcc start to increase again.
Accordingly, the capacitor 17 needs to be given a sufficiently high capacity to prevent the charge voltage Vcc of the capacitor 17 from becoming lower than the minimum operating voltage of the switching control circuit 14 during the period from the time point T1 to the time point T3.
FIGS. 12A to 12C are voltage waveform diagrams illustrating the overcurrent protection operation performed in the switching power supply apparatus. Now, with reference to these voltage waveform diagrams, how the overcurrent protection operation is performed in the switching power supply apparatus will be described.
For example, while the switching power supply apparatus is operating in the steady state, if, at a time point t0 shown in FIGS. 12A to 12C, the output terminals 10 and 11 are short-circuited together because of a fault or the like in the electronic appliance connected as a load to the switching power supply apparatus, the output voltage of the switching power supply apparatus falls sharply as shown in FIG. 12B, and an excessively large current flows through the positive and negative output lines L5 and L6.
This excessively large current is detected by the output current detection circuit 18, which then short-circuits together the cathode of the photodiode 113a and the negative output line L6. This causes the current through the photodiode 113a to increase, and information on this increase in the current is fed, in the form of a feedback signal, through the phototransistor 13b to the switching control circuit 14.
Thus, the switching control circuit 14 recognizes that the output voltage of the switching power supply apparatus has increased, and therefore controls the switching operation of the FET 7 in the direction in which the output power of the switching power supply apparatus decreases. Here, however, the switching operation is not completely stopped until a time point t1, as will be described later.
Specifically, in the period from the time point t0 to the time point t1, when the voltage between the node between the positive output line L5 and the anode of the photodiode 113a and the point on the negative output line L6 at which the output current detection circuit 18 is connected thereto decreases, the currents that flow through the photodiode 113a and the phototransistor 13b decrease, and therefore the switching control circuit 14 controls the switching operation of the FET 7 in the direction in which the output power of the switching power supply apparatus increases commensurately with the decrease in those currents.
On the other hand, when the voltage between the node between the positive output line L5 and the anode of the photodiode 113a and the point on the negative output line L6 at which the output current detection circuit 18 is connected thereto increases, the currents that flow through the photodiode 113a and the phototransistor 13b increase, and therefore the switching control circuit 14 controls the switching operation of the FET 7 in the direction in which the output power of the switching power supply apparatus decreases commensurately with the increase in those currents. Thus, the switching power supply apparatus outputs power of which the level is such that a proper balance is achieved among these conflicting factors.
Moreover, during the period from the time point t0 to the time point t1, since, as described earlier, the positive-direction voltage level of the induced voltage that appears in the auxiliary coil 6c of the transformer 6 is proportional to the output voltage of the switching power supply apparatus, the positive-direction voltage level of the induced voltage is low, and therefore no current is fed through the diode 16 to the capacitor 17.
Accordingly, the start-up current fed through the start-up resistor 15 is smaller than the current consumed by the switching control circuit 14, and thus the charge voltage Vcc of the capacitor 17 decreases gradually until, at the time point t1, it becomes equal to the minimum operating voltage of the switching control circuit 14. Now, the switching control circuit 14 stops operating, and thus the switching power supply apparatus stops switching operation.
In the following period from the time point t1 to a time point t2, the switching control circuit 14 is not operating and thus consumes only a small current. Accordingly, the charge voltage Vcc of the capacitor 17 increases gradually as shown in FIG. 12A owing to the start-up current fed through the start-up resistor 15 until, at the time point t2, it becomes equal to the operation start voltage of the switching control circuit 14. Now, the switching power supply apparatus restarts switching operation.
During this period from the time point t1 to the time point t2, since the switching power supply apparatus is not performing switching operation, as shown in FIGS. 12B and 12C, the output voltage of the switching power supply apparatus is zero, and therefore no included voltage appears in the auxiliary coil 6c of the transformer 6. Thus, no current is fed through the diode 16 to the capacitor 17.
Then, during the period from the time point t2 to a time point t3, as during the period from the time point t0 to the time point t1, the switching power supply apparatus so controls as to output low but fixed power. Accordingly, the positive-direction voltage level of the induced voltage that appears in the auxiliary coil 6c of the transformer 6 is low, and thus no current is fed through the diode 16 to the capacitor 17. As a result, the start-up current fed through the start-up resistor 15 is smaller than the current consumed by the switching control circuit 14, and thus the charge voltage Vcc of the capacitor 17 decreases gradually until, at the time point t3, it becomes equal to the minimum operating voltage of the switching control circuit 14. Now, the switching control circuit 14 stops operating, and thus the switching power supply apparatus stops switching operation.
After the time point t3, as long as the positive and negative output terminals 10 and 11 are short-circuited together, the operations described above that are performed during the period from the time point t1 to the time point t3 are repeated.
When the positive and negative output terminals 10 and 11 cease to be short-circuited, the output current detection circuit 18 cancels the short-circuiting between the cathode of the photodiode 113a of the photocoupler 113 and the negative output line L6. Thus, when a switching operation period (for example, the period from the time point t2 to the time point t3, or the period from a time point t4 to a time point t5 (the time point t5 is not shown in the figure), or any of the succeeding similar switching operation periods) starts, only the current to the shunt regulator 12a of the output voltage detection circuit 12 flows through the photodiode 113a, and this causes the switching power supply apparatus to enter the steady state, in which its output voltage stabilizing function works.
A conventional switching power supply apparatus, like the one described above, that incorporates an overcurrent protection circuit adopting the automatic recover method and that includes a high-capacity power-smoothing capacitor 17 has the disadvantage that, for example when the positive and negative output terminals 10 and 11 are short-circuited together because of a fault in the electronic appliance that is connected as a load to the positive and negative output terminals 10 and 11, the switching power supply apparatus outputs a current larger than the overcurrent protection capability previously set therein. This may cause destruction of the switching power supply apparatus.
Now, the cause of the noted problem will be explained. In this conventional switching power supply apparatus, the capacitor 17 needs to be given a sufficiently high capacitance so that, when the switching power supply apparatus starts to start up, the charge voltage Vcc does not become lower than the minimum operating voltage of the switching control circuit 14, for example, during the period from the time point T1 to the time point T2 shown in FIGS. 11A to 11C.
Otherwise, the charge voltage of the capacitor 17 becomes lower than the minimum operating voltage of the switching control circuit 14 before the positive-direction voltage induced in the auxiliary coil 6c of the transformer 6 becomes higher than the minimum operating voltage of the switching control circuit 14, and thus before the current fed from the auxiliary coil 6c of the transformer 6 through the diode 16 permits the switching control circuit 14 to operate continuously. That is, switching operation is stopped in the middle of start-up operation.
Accordingly, in a case where the capacitance of the power-smoothing capacitor connected between the power input terminals of the load-side appliance (not shown) is high, the extra time required for the switching power supply apparatus to charge the power-smoothing capacitor connected between the power input terminals of the load-side appliance slows down the rate at which the output voltage of the switching power supply apparatus increases, and, since the positive-direction induced voltage that appears in the auxiliary coil 6c of the transformer 6 increases in proportion to the output voltage of the switching power supply apparatus, the rate at which this induced voltage increases is also slowed down. Thus, in a case where the power-smoothing capacitor connected between the power input terminals of the load-side appliance (not shown) is high, it is necessary to give the capacitor 17 a higher capacity so as to extend the time during which the switching control circuit 14 operates from the current discharged from the capacitor 17.
On the other hand, it is also necessary to make relatively short the time required for the switching power supply apparatus to start up so that the user of the apparatus does not feel inconvenience when using it. This time is determined mainly by the time required for the charge voltage of the capacitor 17 to reach the operation start voltage of the switching control circuit 14 owing to the current fed through the start-up resistor 15 (i.e., the period from the time point T0 to the time point T1).
Accordingly, increasing the capacitance of the capacitor 17 inevitably necessitates reducing the resistance of the start-up resistor 15 so as to make the time required for the switching power supply apparatus to start up equal to or shorter than a predetermined length of time.
The conventional switching power supply apparatus shown in FIG. 10 alternately repeats the operations performed during the period from the time point t1 to the time point t2 shown in FIGS. 12A to 12C and the operations performed during the period from the time point t2 to the time point t3 as long as the positive and negative output terminals 10 are short-circuited together. Whereas, during the period from the time point t1 to the time point t2, switching operation is not performed and therefore the switching power supply apparatus consumes almost no power, during the period from the time point t2 to the time point t3, switching operation is performed and so much power is consumed as to output power of which the level is such that a proper balance is achieved among the conflicting factors mentioned earlier, namely the factors that increase the output voltage and those that decreases it, causing mainly the diode 8 and the FET 7 to become hot.
In particular, the voltage between the node between the positive output line L5 and the anode of the photodiode 113a and the point on the negative output line L6 at which the output current detection circuit 18 is connected thereto becomes equal to or higher than the forward voltage drop across the photodiode 113a, and a current equal to this voltage divided by the resistance of the positive and negative output lines L5 and L6 flows through the positive and negative output lines L5 and L6. This resistance is approximately close to zero ohms, and thus produces a short-circuited state. As a result, an excessively large current flows through the diode 8 provided on the positive output line L5, causing the diode 8 to become extremely hot.
Accordingly, to reduce the power consumed by the switching power supply apparatus and to prevent thermal destruction of the diode 8 and other components when the switching power supply apparatus is in the short-circuited state described above, it is necessary to make the period from the time point t2 to the time point t3 shorter relative to the period from the time point t1 to the time point t2.
As described earlier, reducing the resistance of the start-up resistor 15 so as to increase the current fed through the start-up resistor 15 tends to make the period from the time point t2 to the time point t3 longer relative to the period from the time point t1 to the time point t2 for the reason that will be stated later.
Here, to simplify the explanations, it is assumed that a current Ik is constantly fed from the capacitor 5 through the start-up resistor 15 to the capacitor 17, and that the switching control circuit 14 consumes a current Is in a switching operation period (in FIGS. 12A to 12C, the period from the time point t2 to the time point t3, or the period from the time point t4 to the time point t5 (not shown)). Moreover, to make calculations simple, it is assumed that the switching control circuit 14 consumes zero amperes in a no-switching-operation period (in FIGS. 12A to 12C, the period from the time point t1 to the time point t2, or the period from the time point t3 to the time point t4).
Moreover, it is also assumed that the operation start voltage and the minimum operating voltage of the switching control circuit 14 are Eh and EL, respectively, and that the capacitor 17 has a capacitance C. Then, the switching operation period Ton and the no-switching-operation period Toff are given by formula (1) and (2) below.Ton=(Eh−EL)/[C×(Is−Ik)]  (1)Toff=(Eh−EL)/(C×Ik)  (2)
The ratio of the switching operation period Ton to the no-switching-operation period Toff is given by formula (3) below.Ton/Toff=Ik/(Is−Ik)  (3)
This proves that, as the current Ik fed through the start-up resistor 15 to the switching control circuit 14 increases, the period from the time point t2 to the time point t3 becomes longer relative to the period from the time point t1 to the time point t2.
In the short-circuited state described above, the problem of increased power consumption by the switching power supply apparatus and the problem of thermal destruction, possibly resulting from the increased power consumption, of the diode 8 provided on the positive output line L5 are particularly striking in switching power supply apparatuses designed for use worldwide. Typically, switching power supply apparatuses of this type are required to guarantee prescribed performance and safety even if the voltage of the commercially distributed alternating-current power supplied thereto varies, for example, in the range from 85 V to 264 V.
As described earlier, it is necessary to appropriately set the capacitance of the capacitor 17 and then, assuming that the voltage of the commercially distributed alternating-current power is 85 V, set the resistance of the start-up resistor 15 to make the time required for the switching power supply apparatus to start up so short that the user of the apparatus does not feel inconvenience when using it. With these settings, however, when the voltage of the commercially distributed alternating-current power is 264 V, the current fed through the start-up resistor 15 is about three times as large as when the voltage of the commercially distributed alternating-current power is 85 V. This results in an extremely high ratio of the switching operation period to the no-switching-operation period when the load is short-circuited, and thus increases the power consumption by the switching power supply apparatus and the heat dissipation by the diode 8 and other components, causing them to become hot.
Thus, with a conventional switching power supply apparatus incorporating an overcurrent protection circuit as described above, when the voltage of the commercially distributed alternating-current power supplied thereto is high, if the load remains short-circuited for a long time, quite inconveniently, the switching power supply apparatus may be destroyed thermally.
Incidentally, Japanese Patent Application Laid-Open No. H10-304658 discloses a switching power supply apparatus that is so configured as to stabilize the output voltage in a light-load state without the use of a dummy resistor for the purpose of reducing power consumption in a light-load state. This configuration, however, does not solve the aforementioned problems because it is not protected against thermal destruction of circuit components that may result if the load remains short-circuited for a long time when the voltage of the commercially distributed alternating-current power supplied thereto is high.